In rice mills it is necessary to work in a manner avoiding so far as possible breaking the grain, particularly in the stages of husking and polishing or whitening. The polishing or whitening machines now known have certain drawbacks, since for example the emery cone or cylinder machines are very bulky and heavy and require very solid bases which involve special installation and high cost.
The machines for horizontal operation use grain elevators which also complicate and increase the cost of installation of such machines. To avoid the use of grain elevators for feeding and discharge, the machines are installed at successive levels and for the work of inspection stairways are used, which requires greater space for the installation as well as slow and complicated operation. In the compact horizontal machines the polishing or whitening of the grain is harsh and irregular because rotation of the rotor causes intermittent projection of the grains, which are accelerated or decelerated depending on the cycle of elevation or drop.
A further drawback is that existing machines use screens having hexagonal section and file-like teeth set in the edges of the perforations in an irregular manner so that many grains are subject to violent treatment and others to more moderate attack, because they are driven down many times against a screen or shell which is hexagonal or octagonal causing breakage in a high percentage specially of the weaker grains. Breakage of weak grains makes the treatment uneconomic; hence it is necessary to provide a machine which eliminates all the said drawbacks.
The whitening and polishing or pearling machine for grains and cereals and more particularly for rice of the present invention has none of the drawbacks mentioned, since the whitening or polishing of the grain is uniform and every grain is subjected to the same treatment in a smooth and continuous manner. The grains are elevated simultaneously within a vertical drum screen which has an ample separation between the rotor and the screen, so that the grains are subject to the same friction and the product is of uniform quality. Rubbing between one grain and the next is more intense while contact with hard parts such as rotor and screen is moderate, with the advantage that the intensity of the action can be graduated by means of certain knives whose approach can be regulated breaking up the continuity of spinning of the grains, scraping them and holding them and allowing them more or less time according to the whitening or polishing desired.